He was dragged from the icy Moyka River in St Petersburg in the early hours of Saturday morning.
A history professor was arrested on suspicion of murder this weekend after he was dragged from a freezing river carrying two severed arms in his rucksack. After conducting a search at his home, they found Anastasia's decapitated body.
Sokolov, a leading expert on Russian military history, is now in hospital after suffering from hypothermia. One day later, the professor - a man who has been decorated with the Legion of Honor - the highest French order of merit - for his scientific and public works, attempted to dispose of the young woman's body by dismembering it.
The historian, who also taught at Sorbonne University, is the author of books on French emperor Napoleon Bonaparte. Local media reported that he was drunk when he was found.
In 2018, Russian paper Moskovsky Komsomolets published a testimony by a former student of his, which claimed he had beaten and bullied her, once threatening to disfigure her face with a hot iron.
Prof Sokolov reportedly told police that he killed his lover during an argument and then sawed off her head, arms and legs. He was well-known for re-enacting historical battles.
"He has admitted his guilt", Sokolov's lawyer Alexander Pochuev told AFP, adding he regretted what he had done and was now cooperating.
He is said to have planned to get rid of the body before publicly committing suicide dressed as Napoleon. ISSEP has removed Sokolov from his position, releasing a statement on Saturday condemning his actions.
"We learn with horror about the atrocious crime of which Oleg Sokolov is allegedly guilty", it said in a statement on Twitter.
Issep was founded by Marion Maréchal, the niece of far-right National Rally party leader Marine Le Pen and a former lawmaker for the far-right National Front party.